This invention pertains to carbonaceous adsorptive membranes utilized for gas separation, and in particular to methods of passivation to protect such membranes against oxidative degradation in moist air.
Porous carbonaceous membranes separate gas mixtures based upon differing interactions of the molecules in a given gas mixture with the membrane pores. Such membranes can be utilized in continuous processes for gas separation as alternatives to well-known cyclic pressure swing adsorption processes.
One type of porous carbonaceous membrane is a carbon molecular sieve membrane having a microporous structure in which the pore diameters are in the same range as the molecular diameters of the components in the gas mixture. The average pore diameter is controlled during preparation of the membrane so that smaller gas molecules in the mixture pass into the pores while larger gas molecules are excluded from the pores when the gas mixture contacts the membrane. This mechanism is the basis for effecting separation of the gas mixture by molecular sieving based on molecular size relative to carbon pore size. Such membranes are described by J. E. Koresh and A. Sofer in an article entitled "Molecular Sieve Carbon Permselective Membranes. Part 1. Presentation of New Device for Gas Mixture Separation" in Separation Science and Technology 18(8), pp. 723-734, 1983. Carbon molecular sieve membranes are prepared by controlled pyrolysis of polymeric material followed optionally by high temperature oxidation to adjust the average pore diameter. Further methods of preparing such membranes include post-pyrolysis treatment by contacting with air, carbon dioxide, or hydrogen, optionally followed by activation with air, oxygen, carbon dioxide, or water vapor. Such methods are described in UK Patent GB 2 207 666 B.
Another type of porous carbonaceous membrane is an adsorptive carbonaceous membrane having a microporous structure in which the pore diameters are larger than the molecular diameters of the components in the gas mixture, but not large enough to allow significant Knudsen diffusion. The average pore diameter is controlled during preparation of the membrane so that when the prepared membrane is contacted with a gas mixture, more strongly adsorbable molecules in the mixture are preferentially adsorbed and permeate through the membrane in an adsorbed fluid phase to yield a permeate enriched in the more strongly adsorbable molecules. The less strongly adsorbable molecules permeate through the membrane to a lesser extent, and therefore the non-permeate gas is enriched in the less strongly adsorbable molecules. This mechanism is the basis for effecting separation of the gas mixture based on differences in adsorption characteristics. Carbonaceous adsorptive membranes are prepared by controlled pyrolysis of polymeric material to yield the desired pore size distribution described above. Optionally, further high temperature treatment following pyrolysis is carried out in an oxidizing atmosphere to modify the porosity or adsorptive properties of the adsorbent membrane. Such membranes and methods for preparation are described in U.S. Pat. No. 5,104,425.
Carbonaceous molecular sieves in the form of bulk granules are well-known for use in gas separation processes. Such materials are used in fixed-bed pressure swing adsorption processes which operate cyclically in contrast with a continuous process possible with the carbon molecular sieve membrane described above. Granular carbonaceous adsorbents with pores larger than molecular sieve dimensions are well-known for use in pressure swing or vacuum swing adsorption processes, and such processes operate cyclically in contrast with a continuous process possible with the porous carbonaceous adsorptive membrane described above.
Exposure of granular carbon molecular sieve materials to moist ambient air can reduce the effectiveness of such materials in pressure swing adsorption systems, for example those used for the separation of air, as described in a paper entitled "Carbon Molecular Sieves with Stable Hydrophobic Surfaces" by S. K. Verma and P. L. Walker, Jr. in Carbon, Vol. 30, No, 6, pp. 837-844, 1992. It was found that treatment with hydrogen at 5.5 MPa and 150.degree. C. protected the carbon molecular sieve material from degradation caused by exposure to wet air. Treatment with hydrogen or chlorine at higher temperatures did not give satisfactory protection or passivation of the carbon.
The effects of exposing carbonaceous adsorptive membranes to wet air presently are not known in the art.
Carbon molecular sieve membranes and carbonaceous adsorptive membranes described above differ fundamentally in that the former depends critically on the carbon pore size relative to gas molecular sizes to effect gas separation, while the latter depends chiefly on the relative strength of adsorption of the gas molecules when the carbon pore size is in the broad range described above. Because of this fundamental difference, the effect of post-pyrolysis treatment on the gas separation properties of carbon molecular sieve membranes will be distinctly different from the effect on carbonaceous adsorptive membranes.
Post-pyrolysis treatment of carbonaceous adsorptive membranes has the potential for improving the properties and gas separation performance of the membranes. Improved membranes are desirable to reduce capital cost and power consumption in separation systems using such membranes. The methods disclosed in the following specification and defined in the appended claims offer such improvements in the use of carbonaceous adsorptive membranes.